The application generally relates to electrically small antennas (ESAs). The application relates more specifically to ESAs including metamaterial resonant structure to reduce antenna size. The ESA may be mounted on an aircraft for the identification and mapping of subsurface facilities or features.
One object of gathering intelligence data is the identification, mapping, and location of deeply buried underground facilities. The scientific community is interested in methods for locating and mapping underground facilities in non-accessible territory to determine, for example, whether underground nuclear facilities are situated in underground bunkers. A key factor that makes it difficult to detect, locate or map such underground facilities is that conventional radar does not penetrate the Earth's surface. When using conventional radar the electromagnetic waves are reflected and attenuated by the soil, due to the finite conductivity and dielectric loss of the soil.
Typical ground penetrating radar (GPR) may operate in the frequency range of 100-400 MHz, but in that frequency range, the radar can penetrate the Earth's surface to a depth of only about one meter. In order for radar waves to penetrate deeper into the ground, a radar signal with a lower frequency, e.g., in the range of 10-150 kHz, is required. At frequencies as low as 10-150 kHz, the electromagnetic radar wave can penetrate the Earth to a depth as great as 100 meters or more, depending on the soil characteristics. However, since radar antennas are geometrically proportional to the wavelength, operating a radar system at frequencies as low as 10-150 kHz normally requires an enormous antenna. The corresponding wavelengths of 10-150 kHz radiowaves range from 30 km to 3 km. Such an antenna cannot be carried efficiently by an airplane, and in any event may not radiate sufficient power to generate a ground-penetrating radar wave. Further, the resolution of such a low frequency radar system would have limited diffraction properties. Such a radar system would be diffraction limited and able to resolve only those objects or features of sizes comparable to the wavelength. Such relatively large objects or features are much larger than most of the features that are being sought.
These existing GPRs are based on transmitting a very short pulse which includes all of the long wavelength Fourier components and can thus penetrate the ground to some extent. However, such GPRs at best penetrate the ground within about a meter of the Earth's surface. Such GPRs are typically used to locate wires, pipes etc. under the ground within about a meter of the top surface. None of the short pulse GPRs can penetrate to a subsurface depth of about 100 meters, which is the range of depth illumination that is required for detecting strategic underground facilities.
Existing methods for identification and mapping of underground facilities include satellite imagery that can indicate construction or excavation activities on the Earth's surface. Satellite imagery provides an approximate or general location of such a facility. However, many underground facilities are accessible by a rather long tunnel that leads from the excavation point to the final underground destination point, meaning that identifying the entrance point at the surface may provide an inaccurate indication of the location of the underground facility. Depending on the length of the access tunnel, the area to be mapped underground could cover a rather large physical area, on the order of many square kilometers.
Other suggested methods to identify underground facilities require placement of acoustic sensors in the ground to detect activity associated with such underground facilities. Small sensors placed in the vicinity of such a structure may pick up acoustic signatures for identifying the exact location of the facility. However, it is not always possible to place sensors, conceal them from discovery, and then periodically interrogate such sensors in the vicinity of such an underground facility. The underground facilities of interest are often located in restricted areas, e.g., facilities located on foreign territory. Furthermore, it would be necessary to have determined, in advance, at least a general location of such an underground facility. Unless the ground sensors are placed in the exact location where detection of signals is likely, it would be easy to miss detection of the target. Finally, the logistics and cost of placing a large number of sensors make placing acoustic sensors an impractical and unattractive solution.
Electrically small antennas (ESA) are known, such as an electrically small, low “Q” radiator as disclosed in U.S. Pat. No. 6,437,750. However, these ESAs have not been configured to illuminate subterranean images.
The foregoing examples and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon reading of the specifications and study of the drawings. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.